Thick film pastes containing low-melting glass compositions are used to encapsulate electronic devices and circuits and thereby protect them from the environment. Glass compositions suitable for use in such electronic overglaze applications must have a relatively low melting point, must exhibit appropriate thermal expansion, and should not significantly alter the electrical performance of the overprinted electronic materials.
Vasudevan et al., U.S. Pat. No. 5,792,716, disclose a thick film paste that includes a glass composition comprising in weight percent from about 30% to about 60% PbO, from about 5% to about 20% ZnO, from about 2% to about 20% B2O3, from about 4% to about 12% Al2O3, from about 5% to about 18% SiO2, up to about 8% ZrO2, up to about 8% TiO2 and from about 9% to about 21% Nb2O5. Upon firing, the composition according to Vasudevan et al. displays a high level of acid resistance, which is an increasingly important characteristic for electronic overglazes.
Due to concerns regarding the toxicity of lead, there have been substantial efforts in the industry to develop lead-free electronic overglaze compositions. An example of one such lead-free glass composition for use in thick film pastes is disclosed in Hormadaly, U.S. Pat. No. 6,171,987. The glass composition according to Hormadaly includes, in mole percent, 40 to 65% SiO2, 10 to 20% Bi2O3, 0.1 to 3% Al2O3, a total of 15 to 25% glass modifiers, wherein the glass modifiers are 1 to 23% ZnO, 0.1 to 5% CuO, 0.1 to 5 CaO, and 0.1 to 2 MgO, and other glass-forming compounds including at most a total of 5% of TiO2 and/or ZrO2. However, demanding applications, such as surge resistor over glazes, require glass compositions that are much more acid resistant and that crystallize rapidly to minimize interaction with underlying electrical components. The present invention provides a new and improved thick film material for use as an over glaze that provides excellent acid resistance together with rapid crystallization characteristics.